Postnatal development of muscarinic autoreceptors in the rat brain: lateral and medial septal nuclei and the diagonal band of Broca

Abstract

The postnatal development of the release of acetylcholine (ACh) and of presynaptic, release-inhibiting muscarinic autoreceptors was studied in the lateral septum (LS), the medial septum (MS) and the diagonal band of Broca (DB) of the rat brain. To this end, slices (350 μm thick) containing these brain regions from rats of various postnatal ages (postnatal day 3 [P3] to P16, and adult) were pre-incubated with [ 3 H ]choline and stimulated twice (S 1, S 2: 360 pulses, 3 Hz) during superfusion with physiological buffer containing hemicholinium-3 (10 μM). In addition, the activity of choline acetyltransferase (ChAT, in crude homogenates) was determined as a marker for the development of cholinergic functions. At any postnatal age, the electrically-evoked overflow of tritium from slices pre-incubated with [ 3 H ]choline was highest in the DB, followed by the MS whereas in slices containing the LS, it was only small. In all septal regions, the evoked [ 3 H ]overflow was Ca 2+-dependent and tetrodotoxin-sensitive at P3. It increased with postnatal age and reached about 60% of the corresponding adult levels at P16. Presence of the muscarinic agonist oxotremorine (1 μM) during S 2 significantly inhibited the evoked overflow of tritium beginning from P5: no significant effect was detected at P3. The ACh esterase inhibitor physostigmine (1 μM) exhibited significant inhibitory effects from P13 onwards, whereas the muscarinic antagonist atropine (1 μM) did not change the evoked ACh release. The activity of ChAT, as measured for these septal regions at various postnatal ages, correlated well with the [ 3 H ]overflow induced by electrical stimulation. In conclusion, (1) electrically-evoked release of ACh was measured for the first time in three septal subregions; (2) the postnatal development of the presynaptic cholinergic functions: ChAT activity, ACh release and muscarinic autoreceptors occurs almost synchronously in these regions of the septal complex and parallels that in the hippocampal formation; (3) as in the hippocampus, the postnatal development of autoreceptors was delayed with respect to the exocytotic release of ACh.